FIG. 1a of the accompanying drawings illustrates a simple illumination system illustrating a source 11 being concentrated by a lens 12 to meet a target brightness on the illumination area 13. FIG. 1b of the accompanying diagrams also shows a similar system to meet the target 13 but with a reflector 14. The description below also applies to a combined or more complex system comprising reflectors and/or lenses, although these are not illustrated here.
In order to reduce the size of these systems, two modifications can be made. FIGS. 2a and 2b illustrate the first of these modifications. This modification involves simply making the lens 21 or the reflector 22 smaller. The effect of this is that the angle of acceptance of the light from the source 11 will be reduced. Thus the illumination on the target area 13 will be reduced. The source light must either be made brighter (reduced efficiency) or the angular emission of the source must be more collimated 23 in order to meet the target 13.
FIGS. 3a and 3b illustrate the second modification, which is to increase the optical focusing power of the lens 31 and reflector 32 and move the source 33 closer to the optics in order to maintain focus on the target (or collimation in the far field.). The acceptance angle is maintained, but the magnification of the system has now increased because of the reduced source distance. However, the source image 34 on the target or far field has increased so that the brightness has decreased. In order to maintain the target brightness 13, the source light must be made brighter (reduced efficiency) or the source must be made smaller in size 35.
This illustrates that the product of source size and angular emission width predominantly determines the ultimate size for a given efficiency required to meet an illumination target. The actual analysis for real sources is more complex but the same essential result holds true and holds for all linear geometrical optical systems.
This product is called “etendue” and is generally a conserved quantity for these optical systems.
Thus for real lamp and headlight systems, whose efficiency and collimation is important, the size is fundamentally determined by the bulb or LED size and to a lesser extent by the angular distribution. The minimum size is determined by physical limits and for bright lamps, thermal considerations can take precedence. Angular distribution can be controlled somewhat by using integrated reflector systems; however these essentially recycle light back through the source. The effect of this is to create absorption which can reduce the efficiency improvement and to worsen the thermal environment around the source.
For example, a typical high beam section of an LED headlamp can have a system front area in excess of 8000 mm2.
JP 2005/331468 (Sharp, published 2 Dec. 2005) illustrates one method to improve angular distribution by incorporating the light source in a reflector cup and directing the source back towards a parabolic reflector. This system illustrates the method of recycling light and changing the angular profile of the source emission.
JP 2004/241142 (Koito Manufacturing, published 26 Aug. 2004) illustrates a different system whereby a single colour LED is focused onto a phosphor source which is then collimated by a reflector/lens structure. The size of the phosphor is determined by the quality of the focus of the LED, so the etendue is fundamentally still determined by the LED and phosphor emission.
One route to achieve a smaller source size is to use the superior focusing properties of a laser, where the beam focus can be very small. If a small phosphor is placed at the focus point, then a very low etendue source can be produced. This approach is known in the prior art.
JP 7-318998 (Mitsubishi, published 8 Dec. 1995) discloses a laser beam that is transmitted to the lamp by optical fiber and incident on a phosphor bead. A parabolic reflector then collimates the light.
JP 2004-354495 (NEC ViewTechnology, published 16 Dec. 2004) discloses a modification to this system whereby the phosphor is placed on a secondary reflector and the laser beam is collimated onto the phosphor and the emission is directed back onto the larger primary reflector for collimation.
JP 2003-295319 (Nitto Kogaku, published 15 Oct. 2003) discloses an alternative system where the laser is directly collimated by an optical system and the beam is focused through a phosphor with a curved secondary reflector beyond the phosphor to reconcentrate laser light passing through the phosphor.
These systems have fundamental issues which include the fact that the phosphors need to be supported above the reflector and the methods of support will reduce efficiency. In addition, effective cooling of the phosphor is very difficult, limited to convective methods with air or water that are complex and expensive to achieve. The systems also still have isotropic or involve recycled light through the phosphor that will involve some absorption loss.
WO2009/115976 (published 24 Sep. 2009) proposes an automotive front light comprising laterally distributed phosphor elements embedded in a common heatsink. In some embodiments, the output sides of the phosphors communicate with conical reflectors formed in the heatsink.
WO2009/024952 (published 26 Feb. 2009) proposes a spotlight in which a blue or ultraviolet LED source directs light through an optical plate onto a yellow phosphor mounted on the plate and with a heatsink mounted above the phosphor.
US2009/0322205 (published 31 Dec. 2009) proposes a device in which a blue LED illuminates a yellow phosphor to produce white light. A heatsink arrangement is provided in the form of a two dimensional mesh embedded in the phosphor and connected to an external heatsink. There is no optical system for concentrating light from the phosphor.
US2009/322197 (published 31 Dec. 2009) proposes a device is very similar to the device of US2009/0322205 except that the mesh embedded within the phosphor is omitted and the edge of the phosphor is thermally coupled to a metal housing which is thermally coupled to a heatsink.
US2004/0159900 (published 19 Aug. 2004) proposes a device in which a blue LED excites a yellow or red and green phosphor to produce white light, or in which an ultraviolet LED excites a red, green and blue phosphor. Both the LED and the phosphor may be mounted so as to be coplanar on a heatsink and an ultraviolet reflector may be disposed above both so as to reflect ultraviolet radiation from the LED onto the phosphor.